Most DC-AC signal sources, such as ringing generators commonly used in telecommunication systems, require a stable, highly accurate sine wave signal source. A sine wave can be generated using logarithmic op-amps or other non-linear forms of signal integration, but sine waves generated in this manner generally have a noticeable amount of distortion. Sine waves can also be generated digitally using a relatively expensive digital to analog converter.
Sine wave generators using multiplexers or counters (and sometimes both) are known. U.S. Pat. No. 3,863,158, for example, discloses a multiplexer receiving 16 preset voltages, wherein a clock signal is used to drive the multiplexer to output the preset voltages to simulate a sine wave. On the other hand, U.S. Pat. No. 4,543,326 discloses a multiplexer having outputs connected to a resistor network. The multiplexer outputs are selected in response to signals from a five bit counter which counts in only one direction and which uses the most significant bit of the counter to determine the polarity of the output section of the sine wave.
Other methods for generating sine waves are disclosed in exemplary U.S. Pat. Nos. 3,500,213, 3,992,680, 4,056,692, 4,061,909, 4,368,432, 4,446,436, 4,710,890, 4,727,570 and 4,751,402.
U.S. Pat. No. 3,500,213, for example, discloses a sine wave synthesizer which employs an up/down counter connected to a decoder circuit. The output of the decoder circuit is provided to an amplifier via a D/A converter. Another exemplary patent, U.S. Pat. No. 3,992,680, discloses a precision frequency generator for producing a frequency proportional to flow rate. The frequency generator includes an oscillator coupled to a binary counter, which provides intermediate frequency pulses, which are a multiple of both the input and output frequencies. In particular, a switch connected between the output of the counter and a triangle generator, via a four-bit binary counter and a pair of gate logic units, is described. The triangle waveform is converted to a sine wave by a conventional converter.
Additionally, U.S. Pat. No. 4,056,692 discloses a digital tone generator including a pair of ring counters connected to a voltage divider network, from which the output signal is taken. In particular, a selection pad selects two signals having different frequencies, which are applied to a plurality of AND gates controlled by a pair of binary counters. The outputs of the pair of AND gates are provided to corresponding one shot multivibrators, which, in turn, control corresponding ring counters.
U.S. Pat. No. 4,061,909 discloses a variable waveform synthesizer including a binary counter driving a decoder network coupled to a plurality of current sources via a combining network of logic gates. Each of the current sources is connected via a summing resistor to an integrator, which produces the sine wave output by averaging the input pulses.
U.S. Pat. No. 4,368,432 discloses a sine wave generator including an oscillator, a frequency divider and a counter. The counter outputs are connected to a low pass filter via a resister network. U.S. Pat. No. 4,446,436 discloses an analog signal generating circuit employing a digital logic network to control a plurality of transistors connected to a multitap resistor, while U.S. Pat. No. 4,710,890 discloses a balanced mixer including an exclusive OR network connected to a plurality of resistors and driven by a Johnson counter.
Another method for generating waveforms is disclosed by U.S. Pat. No. 4,727,570, for example, wherein a waveform generator including a D/A converter driven by a pair of four bit counters is shown. In particular, the waveform generator counters are controlled by a logic gate responsive to frequencies produced by a column sine wave generator and a row sine wave generator. U.S. Pat. No. 4,751,402, on the other hand, discloses a complex signal generator using linear approximations including a counter coupled to a switching network via a binary to decimal decoder.
The present invention was motivated by a desire to produce a low frequency sine wave generator employing a small number of components but capable of producing a symmetrical low distortion waveform.